The invention relates to a device for controlling the pitch of the blades of a convertible aircraft rotor of the known type supporting at least two wings, generally fixed, and at least one tilting rotor.
A particularly advantageous application of the pitch control device according to the invention consists in fitting it to constant velocity drive tilting rotors for convertible aircraft, particularly of the type known and described in U.S. Pat. Nos. 6,276,633, 6,260,793 and 6,607,161 to which reference may advantageously be made for further details.
Briefly, such a convertible aircraft with tilting rotors generally comprises, as shown schematically in FIG. 1, a fuselage 1, of the aircraft fuselage type, supporting two fixed wings 2, for example high wings, raised with respect to the fuselage 1, each wing 2 itself supporting a power pod 3, housing a power unit driving in rotation a rotor 4, represented schematically by the plane of rotation of the rotor blades, via a transmission (not shown in FIG. 1), a rear reduction gear unit of which is directly driven by the power unit and housed like the latter in the stationary rear part 5 of the power pod 3. The front part 6 of the power pod, housing a rotor mast and a rotor hub, as well as a front reduction gear unit driving the rotor mast in rotation, is mounted so as to pivot with the rotor 4, so that it can pivot from an aeroplane configuration, in which the rotor 4 operates as a propeller at the front of an engine pod 5-6 facing into the relative wind, to a helicopter configuration, in which the rotor 4xe2x80x2 operates as a helicopter main lifting rotor at the upper end of the front pivoting part of the pod in the upright position 6xe2x80x2 above the corresponding wing 2, this latter configuration being used for take-off and landing of the convertible aircraft which, after transition from the helicopter configuration to the aeroplane configuration, is able to move in forward flight like an aircraft. As a variant, the pods 3 may pivot totally with the rotor 4 with respect to the fixed wings 2.
The tilting rotors of convertible aircrafts differ from the rotors, particularly main rotors, of helicopters by the following characteristics. The range of control of the blades of these rotors in collective pitch is very large, due to the fact that the aircraft must be able to fly not only as a helicopter but also as an aeroplane; the power for controlling them must remain low, which necessitates a very small flapping eccentricity (i.e. the distance between the flapping axis of the blades and the axis of rotation of the rotor); the rotors, and their hubs in particular, must have high rigidity in the plane of the rotor, in order to obtain a high drag mode, to avoid the dangers of ground resonance; and they have a high level of cyclic flapping.
To meet these different requirements, it has already been proposed that such convertible aircrafts be equipped with tilting rotors of the gimbal type, with zero flapping eccentricity, on which the hub is driven in rotation by the mast and connected to the latter by tilting means, allowing the hub, and therefore the rotor, to tilt as a whole about a flapping axis passing through the axis of the mast and perpendicular to the latter, and being able to occupy any position extending in any direction about the axis of the mast. On such rotors, the absolute requirement that the pitch-flap coupling, commonly referred to as the delta-3 relation, should have a slightly negative value for the control of such a tilting rotor, has led the designers of such rotors to adopt three-bladed rotors, so that a conventional pitch control device is retained, similar to those fitted to helicopter main rotors, with a swashplate mechanism, in which the oscillations of a rotating star control the movements, substantially along the axis of rotation of the rotor mast, of pitch control rods, each of which is hinged to the end of a pitch change lever integral in rotation with the root of a corresponding blade about the longitudinal pitch change axis of this blade, the pitch change lever projecting on the leading edge or trailing edge side of this blade, in order that the distance between the flapping axis and the centre of the pitch change lever hinge to the corresponding pitch control rod is short.
In actual practice, this condition of a short distance between the flapping axis and the centre of the pitch change lever hinge to the control rod can be met on a three-bladed rotor with conventional pitch change levers, i.e. levers placed substantially in the plane of the rotor disc, since the space available between successive blade roots is generally sufficient to accommodate such pitch change levers.
On the other hand, on a rotor equipped with at least four blades, the flapping axis of a blade corresponds substantially with the longitudinal axis of the following blade, so that for reasons of geometry and interference the pitch change levers cannot be positioned to satisfy the above conditions.
The problem addressed by the present invention is to propose a device for controlling the pitch of the blades of a gimbal-type rotor as defined above, preferably rigid in plane, and which may comprise at least three blades, but preferably four blades or more, while meeting the strict requirement for a slightly negative delta-3 value.
To this end, there is provided a device for controlling the pitch of blades of a convertible aircraft tilting rotor, for a rotor comprising:
a rotor mast having a base and a longitudinal axis, the rotor mast being capable of being driven in rotation by said base about said longitudinal axis,
a hub, driven in rotation by the mast, and connected to the masse by a tilting arrangement, allowing the hub as a whole to tilt about any flapping axis passing through the axis of the mast and perpendicular to the axis of the mast, and
at least two blades, each of which is linked to the hub by a coupling retaining and hingeing said blade in pitch about a longitudinal pitch change axis of said blade, which pitch change axis extends in a plane of rotation of the rotor,
wherein the pitch control device is external to the hub and comprises for each blade of the rotor:
a pitch change lever, integral in rotation with a root of said blade about said pitch change axis, and projecting on a side of a plane of rotation of the rotor which faces towards the base of the mast,
a pitch control rod which is movable in a direction substantially parallel to the longitudinal axis of rotation of the mast,
a bellcrank mounted so as to pivot about a pivot axis substantially contained in a radial plane relative to the longitudinal axis of rotation of the mast, on a fixed point of a drive plate, integral in, rotation with the mast, said pitch control rod being hinged to said bellcrank, at least so as to pivot about an axis substantially parallel to the pivot axis of the bellcrank on the drive plate, and at a point offset radially outwardly of said pivot axis relative to the longitudinal axis of rotation of the mast, and
a link rod, hinged at least so as to pivot at one end to said pitch change lever, about an axis substantially parallel to said pitch change axis and, at an other end of said link rod to the bellcrank, at a fixed point on the bellcrank located between said plane of rotation of the rotor and the point of hingeing of the bellcrank to the drive plate, so that said link rod is slightly inclined to any radial plane relative to the longitudinal axis of rotation of the mast, and that the bellcrank converts any movement of said pitch control rod into movement of said link rod in a direction substantially transverse to the direction of the longitudinal axis of the mast.
An essential advantage of this pitch control device is that an angle of inclination of said link rod to any radial plane (perpendicular to the axis of the mast) may be chosen to correspond to a desired pitch-flap relation (delta-3) for the corresponding blade.
Also advantageously, said link rod remains one of substantially parallel to and slightly inclined to any radial plane relative to the axis of rotation of the mast, when said blade performs angular deflection movements within its pitch range.
Moreover, the angle of inclination of said link rod to any radial plane, perpendicular to the axis of the mast, may vary so as to obtain pitch-flap coupling (delta-3) according to a law determined as a function of pitch angle of the blade, when the latter performs angular deflection movements in flapping.
In an advantageously simple form of structural embodiment, the bellcrank is substantially L-shaped, hinged at the elbow to the pitch control rod, at one end of one of its arms to the drive plate, and at one end of its other arm to the link rod. The overall size of the bellcrank is thus limited, while allowing satisfactory positioning of the pitch control rod and link rod hinges on the bellcrank, and of the latter on the drive plate, so as to obtain the slight inclination desired of the link rod to the radial plane, and therefore the desired delta-3 relation.
The link rod may thus, for different flapping values, and as a function of the pitch of the corresponding blade, exhibit a change in its inclination relative to any radial plane relative to the axis of the mast which has the effect that this inclination, the slope of which with respect to the radial plane (relative to the axis of the mast) has an effect on the delta-3 relation obtained, remains within a limited range of angular values, less than 20xc2x0, for a pitch range extending substantially from xe2x88x9210xc2x0 to 60xc2x0, and flapping values varying from xe2x88x9211xc2x0 to +11xc2x0.
This slight inclination of the link rod to any radial plane relative to the axis of the mast, and in a substantially transverse direction relative to the direction of the axis of the mast, is also favourable in terms of size, as well as in terms of final adjustments, since easily implemented changes in the positions of the bellcrank and the link rod, and in particular of their hinges to each other, and also to the pitch change lever for the link rod, and to the pitch control rod and the drive plate, for the bellcrank, make it easy to test different configurations of the pitch-flap relation, in order to obtain the desired delta-3 value by appropriate inclination of the link rod.
Due to the distance between the hinge of the link rod to the pitch change lever and the flapping axis, this inclination varies with the cyclic flapping, which induces a pitch-flap relation which depends on the cyclic flapping. However, as this variation in inclination is opposite in sign and substantially equal in amplitude for two diametrically opposite blades, for example in a four-bladed rotor, the result is a pitch-flap relation for the cyclic mode which is substantially constant, when considered relative to the tilting hub and to the rotor as a whole. In concrete terms, a disturbance in cyclic flapping for a rotor subjected to substantial cyclic flapping loads causes a different variation in pitch for two opposite blades, but consideration as a whole of the variation in aerodynamic moment applied at the centre of the rotor head shows that this moment remains very substantially proportional to the disturbance in cyclic flapping, and independent of cyclic flapping.
The arrangement of this pitch control device with, for each blade, a link rod substantially in the radial plane relative to the axis of the mast, is more particularly suited to a rotor rigid in drag, for which drag movement of the blades is very slight, which limits the pitch variations induced by the pitch-drag coupling. The limitation of the pitch variations induced by this pitch-drag coupling may be optimised by arranging the end of the link rod hinged to the corresponding pitch change lever in the plane perpendicular to the pitch hinge axis and containing the vibration node of the cyclic drag mode.
In addition, the arrangement of the link rod, projecting from the leading edge or from the trailing edge of the corresponding blade, starting from the pitch change lever of this blade, may be chosen according to the desired sign of the pitch-drag coupling or relation for the cyclic and collective drag modes.
Also advantageously, to avoid unwanted types of coupling as far as possible, the pivot axes of the bellcrank respectively on the drive plate, on the pitch control rod, and on the link rod are substantially parallel with each other, and also to the pitch change axis of the corresponding blade, in the neutral position of the hub.
In general terms, the bellcrank and the link rod, and also preferably the pitch control rod, move substantially in a plane parallel to the longitudinal axis of the mast and perpendicular to the pivot axis of the bellcrank on the drive plate and substantially perpendicular to the pitch change axis of said blade.
Also advantageously, to keep a small delta-3 value with low eccentricity and any number of blades, the pitch change lever projects towards the base of the mast at a radially inner end, relative to the axis of the mast, of a blade cuff, rigid in torsion and connected in torsion to the root of the corresponding blade about the pitch change axis, and surrounding said coupling for retaining and hingeing said blade in pitch. In this way, the pitch control takes place close to the hub and to the axis of the mast, and this pitch control device is directly compatible, because of the presence of the cuffs, with rotor hubs of the type with a plate in the form of a star comprising as many outward-extending arms as the rotor has blades, each arm of the hub supporting the coupling for retaining and hingeing a corresponding blade in pitch, these hubs being known for providing good rigidity in drag while exhibiting low rigidity in flapping, by the bending of the arms of the hub perpendicularly to their plane.
To ensure proper guidance of the bellcrank in its pivoting points on the drive plate, and therefore also corresponding proper guidance of the ends of the pitch control rod and of the link rod which are hinged to this bellcrank, the latter is advantageously mounted so as to pivot in a yoke projecting outwardly on the drive plate. On the other hand, the link rod is advantageously hinged to the pitch change lever and to the bellcrank by ball joint connections, and/or the bellcrank is hinged to the pitch control rod by a ball joint connection.